N-substituted monomers and polymers

ABSTRACT

Biocompatible, bioresorbable polymers comprising a plurality of monomeric repeating units containing an amide group, wherein said amide groups are N-substituted and the N-substituent and degree of N-substitution are effective to lower the melt viscosity, the solution viscosity, or both, compared to the same polymer without N-substitution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/873,979, filed on Oct. 17, 2007, now U.S. Pat. No. 8,008,528, whichclaims priority under 35 U.S.C. §119(e) to U.S. Provisional PatentApplication Ser. No. 60/852,471, filed on Oct. 17, 2006 and entitled“N-Substituted Monomers and Polymers,” and also claims priority under 35U.S.C. §120 to International Application No. PCT/US07/81571 designatingthe United States and filed on Oct. 16, 2007. The contents of theforegoing applications are hereby incorporated by reference in theirentireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to N-substituted monomers and polymers, methodsof making such monomers and polymers, and methods of using them invarious applications, such as medical devices.

2. Description of the Related Art

The tyrosine-derived monomers of U.S. Pat. No. 5,099,060 polymerize toform polymers with higher melt or solution viscosities that may resultin poor processibility. As a result, the fabrication of the polymersrequires higher temperatures, higher pressures, or both, that are lesseconomical and may also degrade the polymer or any additives (such asbiological or pharmaceutical moieties).

Such higher melt or solution viscosities can occur with tyrosine-derivedpolymers such as the polyiminocarbonates of U.S. Pat. No. 4,980,449, thepolycarbonates of U.S. Pat. No. 5,099,060, the polyarylates of U.S. Pat.No. 5,216,115, the poly(alkylene oxide) block copolymers of U.S. Pat.No. 5,658,995, the phosphorous-containing polymers of U.S. Pat. Nos.5,912,225 and 6,238,687, the anionic polymers of U.S. Pat. No.6,120,491, the poly(amide carbonates) and poly(ester amides) of U.S.Pat. No. 6,284,862, the radio-opaque polymers of U.S. Pat. No.6,475,477, and the polyethers of U.S. Pat. No. 6,602,497. Thedisclosures of all the foregoing patents are incorporated herein byreference in their entirety.

There exists a need for polymers with lower melt viscosities that arecapable of being melt-processed and/or solution processed with greaterease, lower temperatures and/or pressures.

SUMMARY OF THE INVENTION

This need is met by the present invention. It has now been discoveredthat the amide bonds present in tyrosine-derived biocompatible polymersare involved in inter-chain hydrogen bonding, which can interfere in thethermal processibility of the polymer because hydrogen bonding betweenpolymer chains increases melt or solution viscosity. In turn this haslead to the discovery that the effect due to hydrogen bonding inmonomers and polymers with peptide linkages can be significantly reducedby replacing the hydrogen atom on the amide nitrogen with methyl orother alkyl groups.

It has surprisingly been discovered that replacing the amide hydrogenwith a non-hydrogen-bonding substituent eliminates or greatly reducesthis source of intermolecular interaction to a degree such that polymersolubility in organic solvents increases, melt viscosity decreases, andthe polymer glass transition likewise decreases. These changes inpolymer properties can be so profound that some polymers that wereinitially non-processible can now be processed by a variety offabrication technologies, including solvent casting, wet and meltspinning, compression molding, extrusion, and injection molding.

Consequently, an N-substituted version of the polymer may be processedat lower temperatures (e.g., relative to the polymer glass transitiontemperature or T_(g)) with less thermal/oxidative degradation. Thisopens the temperature processing window for the polymer, e.g., higherT_(g) polymers can be processed at existing process temperatures andsimilar T_(g) polymers may be processed at lower temperatures.

Likewise, polymers solvated in relatively non-polar solvents, such asdichloromethane, can be processed at higher solids concentrations withlower solution viscosities.

Therefore, according to one aspect of the present invention,biocompatible, bioresorbable polymers are provided comprising aplurality of monomeric repeating units containing an amide group,wherein the amide groups are N-substituted and the N-substituent anddegree of N-substitution are effective to lower the melt viscosity, thesolution viscosity, or both compared to the same polymer withoutN-substitution. According to one embodiment, the N-substituents anddegree of N-substitution are effective to reduce the melt viscosity, thesolution viscosity, or both, at least about 5%, and in anotherembodiment the reduction is at least about 10%. According to anotherembodiment, the N-substituents are C₁-C₆ alkyl groups. According to yetanother embodiment, the N-substituent is a methyl group. According toanother embodiment, the present invention includes polymers with one ormore recurring units of formula (I):

wherein X¹ and X² in formula (I) are each independently selected from Brand I; y1 and y2 in formula (Ia) are each independently zero or aninteger in the range of 1 to 4, and R¹ is selected from substituted orunsubstituted, saturated or unsaturated, straight chain or branchedaliphatic groups containing up to 48 carbon atoms, substituted orunsubstituted aromatic groups containing up to 48 carbon atoms, andsubstituted or unsubstituted araliphatic groups containing up to 48carbon atoms in which the aliphatic portions are straight chain orbranched and saturated or unsaturated, and R¹ contains from 2 to 8heteroatoms selected from O, S and N, in which two of the heteroatomsform a polymer backbone amide group that is N-substituted. Additionalheteroatoms are present when the R¹ group contains poly(alkylene oxide)groups.

Unless otherwise defined for a specific embodiment, N-substituted aminesare substituted with a substituted or unsubstituted, straight orbranched, saturated or unsaturated aliphatic group containing up to 30carbon atoms, a substituted or unsubstituted aromatic group containingup to 30 carbon atoms, and a substituted or unsubstituted araliphaticgroup containing up to 30 carbon atoms in which the aliphatic portion isstraight chain or branched and saturated or unsaturated. According toone embodiment, R¹ groups contain between about 18 and about 36 carbonatoms. According to another embodiment, the N-substituents are C₁-C₆alkyl groups. According to yet another embodiment, the N-substituent isa methyl group.

According to one embodiment, R¹ has a pendant carboxylic acid group or apendant carboxylic acid ester or N-substituted amide. According to anembodiment R¹ has a pendant N-substituted tertiary amine. According toone embodiment, R¹ has both a pendant carboxylic acid group or a pendantcarboxylic acid ester or N-substituted amide and a pendant N-substitutedtertiary amine. According to another embodiment, R¹ in formula (I) is:

in which R¹³ and R¹⁴ each independently contain from 0 to 8 carbonsatoms, inclusive, and are independently selected from(—CHR¹)_(e)—CH═CH—(CHR¹—)_(e) and (—CHR¹)_(f)(—CHNQ²)_(g)(—CHR¹)_(f),wherein R¹ is H or lower alkyl, each e independently ranges between 0and 6, inclusive, each f independently ranges between 0 and 8, inclusiveand g is 0 or 1; Z⁷ is O or S; R^(x) is selected from optionallysubstituted branched or unbranched C₁-C₃₀ alkyl and optionallysubstituted C₆-C₃₀ aryl; Q¹ is C(═Z⁵)—R⁸, wherein Z⁵ is 0 or S; Q² is—N(R^(x))₂ or N(1VQ¹); R⁸ is selected from H, a therapeutically activemoiety, a poly(alkylene oxide), X₃-C_(i)-C₁₈ alkyl, X₃-alkenyl,X₃-alkynyl, —X₅-cycloalkyl, —X₅-heterocyclyl, —X₅-aryl and—X₅-heteroaryl;

X₃ is selected from a bond, O, S, and N-alkyl; X₄ is selected from O, Sand N-alkyl; and X₅ is selected from a bond, lower alkyl, O, S andN-alkyl.

Alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclyl, aryl and heteroarylgroups, except when otherwise defined, contain up to 30 carbon atoms.According to an embodiment, the groups contain up to 18 carbon atoms.Lower alkyl groups, except when otherwise defined, are straight orbranched and contain up to 6 carbon atoms. Alkyl, alkenyl and alkynyl,groups are also straight or branched and contain from 0 to eightheteroatoms, and lower alkyl groups also contain 0, 1 or 2 heteroatoms.Heteroatoms are independently selected from O, S and N-lower alkyl.Heterocyclyl and heteroaryl groups also contain from one to eightheteroatoms selected from O, S and N-lower alkyl.

According to one embodiment the poly(alkylene oxide) R⁸ groups includealkyl-terminated poly(alkylene oxides) of molecular weight 100 to10,000, examples of which include methoxy-terminated poly(ethyleneglycols) (PEG), methoxy-terminated poly(propylene glycols) (PPG), andmethoxy-terminated block copolymers of PEG and PPG. According to anotherembodiment poly(alkylene oxide) groups have a molecular weight betweenabout 400 and about 4000. According to another embodiment thepoly(alkylene oxides) are poly(ethylene glycols) with molecular weightsbetween about 1000 and about 2000.

According to another embodiment, one or both aromatic rings may besubstituted with from 1 to 4 groups independently selected from halogen,lower alkyl, carboxyl, nitro, thioether, sulfoxide and sulfonyl as longas the substitution patterns are chemically feasible. Any combination ofsubstituents containing more than two nitro substituents is potentiallyexplosive and expressly excluded from these teachings. Monomers andpolymers with a sufficient number of aromatic rings sufficientlysubstituted with bromine or iodine are inherently radio-opaque. Inpreferred radio-opaque monomers and polymers, at least one monomericaromatic ring is substituted with iodine, so that the sum of y1 and y2in formula (I) is greater than zero, preferably on at least one and morepreferably on both ring positions ortho to the phenolic oxygen.Preferably both aromatic rings are iodine-substituted at both orthopositions.

According to yet another embodiment, R¹ in formula (I) is selected from:

wherein R^(x) and R⁸ are the same as described above with respect toformula II; a and b range from 0 and 8, inclusive, and Z⁴ and Z⁵ areeach independently O or S. According to more specific embodiments, a=1and b=2.

Polymers according to the present invention include polycarbonates,polyarylates, polyiminocarbonates, polyphosphazenes andpolyphosphoesters having the structure of formula (Ia),

wherein X¹, X ², y1, y2 and R¹, and the embodiments thereof, are thesame as described above with respect to formula (I) and A¹ is selectedfrom:

wherein R¹° is selected from H, C₁-C₃₀ alkyl, alkenyl or alkynyl andC₂-C₃₀ heteroalkyl; heteroalkenyl or heteroalkynyl, and R¹² is selectedfrom C₁-C₃₀ alkyl, alkenyl or alkynyl, C₁-C₃₀ heteroalkyl; heteroalkenylor heteroalkynyl, C₅-C₃₀ heteroalkylaryl, heteroalkenylary orheteroalkynylaryl, C₆-C₃₀ alkylaryl, alkenylaryl or alkynylaryl, andC₅-C₃₀ heteroaryl.

In an embodiment, R^(x) is a branched or unbranched C₁-C₆ alkyl. In aspecific embodiment, R^(x) is methyl. In an embodiment, Q¹ is a grouphaving the structure:

wherein t in the above groups is independently in the range of zero toabout 18.

A polymer comprising a recurring unit of formula (I) can becopolymerized with any number of other recurring units. In anembodiment, the polymer comprising a recurring unit of formula (I)further comprises a recurring polyalkylene oxide block units of theformula (III):

wherein B is —O—((CHR⁶)_(p)—O)_(q)—; each R⁶ is independently H or C₁ toC₃ alkyl; p is an integer ranging between about one and about four; q isan integer ranging between about five and about 3000; and A² is the sameas A¹ in formula (Ia). One block copolymerized polymer embodimentcontains a molar fraction of alkylene oxide between about 0.1 and about25%. Another embodiment contains a molar fraction of alkylene oxidebetween about 0.5 and about 10%. Yet another embodiment contains a molarfraction of alkylene oxide between about 1 and about 5%.

N-substituted polymers according to the present invention arepolymerized from diphenols corresponding to the structure of formula (I)prepared according to the methods disclosed by the above-referenced U.S.Pat. No. 5,099,060, the entire disclosure of which is incorporatedherein by reference. The polymers can be copolymerized with diphenolsthat are not N-substituted. Polymers according to the present inventioncontain embodiments in which the molar fraction of N-substituted monomeris between about 1 and about 50%. Another embodiment provides polymerswith a molar fraction of N-substituted monomer between about 5 and about25%. Yet another embodiment provides polymers with a molar fractin ofN-substituted monomer between about 7.5 and about 12.5%.

N-substituted diphenol compounds thus represent new and useful compoundsaccording to the present invention. The present invention therefore alsoincludes diphenol compounds with amide groups that are N-substituted.One embodiment includes diphenol compounds in which the N-substituent isa C₁-C₆ alkyl group. Another embodiment includes diphenol compoundshaving the structure of formula (IV):

wherein X¹, X ², y1, y2 and R¹, and the embodiments thereof, are thesame as described above with respect to formula (I).

According to one diphenol embodiment, R¹ is selected so the Formula IVmonomer is an N-substituted dityrosine such as the N,N-dimethyldityrosine depicted below formed by N-methylation of the dityrosinedepicted below:

Dityrosines and their preparation are reported in the literature, anddityrosines can be N-substituted by the procedures disclosed herein. Thepresent invention also includes Formula I and Formula Ia polymerspolymerized from the N-substituted dityrosines of the present invention.

In general, polymers according to the present invention possessexcellent physical properties and melt processability and can be shapedinto different three-dimensional structures for specific uses byconventional polymer-forming techniques such as extrusion and injectionmolding. The solvent-casting and compression molding techniquesdescribed in earlier patents disclosing polymers polymerized fromtyrosine-derived diphenol compounds can also be used. Therefore,according to another aspect of the present invention, blood-contactingor tissue-implantable medical devices are provided, formed from thepolymers of the present invention. Preferably, the devices are formed bythermal fabrication. Such devices include hernia repair devices.

According to one embodiment of this aspect of the invention, the medicaldevice is a stent for treatment of a body lumen. Preferred stents areformed from or coated with radio-opaque polymers according to thepresent invention, so that fluoroscopic imaging can be used to guidepositioning of the device. A preferred radio-opaque, bioresorbable stentaccording to one embodiment of the present invention is formed from abioresorbable polymer with sufficient halogen atoms to render the stentinherently visible by X-ray fluoroscopy during stent placement.

According to another aspect of this embodiment of the present invention,the medical device is an embolotherapy product. Embolotherapy productsaccording to the present invention are particulate formulations ofbiocompatible, bioresorbable polymers according to the presentinvention. In a preferred embodiment, the polymer contains a sufficientnumber of halogen atoms to render the embolotherapy product inherentlyradio-opaque.

Other specific applications for which the polymers of the presentinvention are also particularly useful include scaffolds for tissueengineering on which isolated cell populations may be transplanted inorder to engineer new tissues. The polymers are formed into porousdevices as described by Mikos et al., Biomaterials, 14, 323-329 (1993)or Schugens et al., J. Biomed. Mater. Res., 30, 449-462 (1996) or U.S.Pat. No. 6,103,255 to allow for the attachment and growth of cells asdescribed in Bulletin of the Material Research Society, Special Issue onTissue Engineering (Guest Editor: Joachim Kohn), 21(11), 22-26 (1996).Therefore, another aspect of the present invention provides a tissuescaffold having a porous structure for the attachment and proliferationof cells either in vitro or in vivo formed from polymers according tothe present invention.

Another specific application includes implantable drug delivery deviceswhere a pharmaceutically active moiety is admixed within the polymericmatrix for slow release, including devices for ophthalmic drug delivery.Therefore, in one embodiment of the present invention, the polymers arecombined with a quantity of a biologically or pharmaceutically activecompound sufficient to be therapeutically effective as a site-specificor systemic drug delivery system as described by Gutowska et al., J.Biomater. Res., 29, 811-21 (1995) and Hoffman, J. Controlled Release, 6,297-305 (1987). Furthermore, another aspect of the present inventionprovides a method for site-specific or systemic drug delivery byimplanting in the body of a patient in need thereof an implantable drugdelivery device containing a therapeutically effective amount of abiologically or a physiologically active compound in combination with apolymer of the present invention.

Polymers in accordance with the present invention may be prepared havinggood film-forming properties. An important phenomena observed for thepolymers of the present invention having poly(alkylene oxide) blockcopolymer segments is the temperature dependent phase transition of thepolymer gel or the polymer solution in aqueous solvents. As thetemperature increases, the gel of the polymers undergo a phasetransition to a collapsed state, while polymer solutions precipitate ata certain temperature or within certain temperature ranges. The polymersof the present invention having poly(alkylene oxide) segments, andespecially those that undergo a phase transition at about 30 to 40° C.on heating can be used as biomaterials for drug release and clinicalimplantation materials. Specific applications include films and sheetsfor the prevention of adhesion and tissue reconstruction.

Therefore, in another embodiment of the present invention, poly(alkyleneoxide) block copolymers of polymers according to the present inventionmay be formed into a sheet or a coating for application to exposedinjured tissues for use as barrier for the prevention of surgicaladhesions as described by Urry et al., Mat. Res. Soc. Symp. Proc., 292,253-64 (1993). Therefore, another aspect of the present inventionprovides a method for preventing the formation of adhesions betweeninjured tissues by inserting as a barrier between the injured tissues asheet or a coating of the radio-opaque poly(alkylene oxide) blockcopolymers of polymers according to the present invention.

The poly(alkylene oxide) segments decrease the surface adhesion of thepolymers of the present invention. As the molar fraction ofpoly(alkylene oxide) increases, the surface adhesion decreases. Polymercoatings containing poly(alkylene oxide) segments according to thepresent invention may thus be prepared that are resistant to cellattachment and are useful as non-thrombogenic coatings on surfaces incontact with blood. Such polymers also resist bacterial adhesion in thisand in other medical applications as well. The present inventiontherefore includes blood contacting devices and medical implants havingsurfaces coated with the poly(alkylene oxide) block copolymers of thepresent invention.

The coated surfaces are preferably polymeric surfaces. Methods accordingto the present invention include implanting in the body of a patient ablood-contacting device or medical implant having a surface coated withthe polymers of the present invention containing poly(alkylene oxide)block copolymer segments.

By varying the molar fraction of poly(alkylene oxide) segments in theblock copolymers of the present invention, the hydrophilic/hydrophobicratios of the polymers can be attenuated to adjust the ability of thepolymer coatings to modify cellular behavior. Increasing levels ofpoly(alkylene oxide) inhibits cellular attachment, migration andproliferation, while increasing the amount of pendent free carboxylicacid groups promotes cellular attachment, migration and proliferation.Therefore, according to yet another aspect of the present invention, amethod is provided for regulating cellular attachment, migration andproliferation by contacting living cells, tissues, or biological fluidscontaining living cells with the polymers of the present invention.

Through pendant free carboxylic acid groups, derivatives of biologicallyand pharmaceutically active compounds, including drugs, can be attachedto the polymer backbone by covalent bonds linked to the carboxylic acidpendent chain. This provides for the sustained release of thebiologically or pharmaceutically active compound by means of hydrolysisof the covalent bond between the drug and the polymer backbone. Thepresent invention therefore also includes polymer embodiments in which Ris a biologically or pharmaceutically active compound covalentlyattached to the polymer backbone.

In addition, polymers of the present invention with pendent carboxylicacid groups have a pH dependent dissolution rate. This further enablesthe polymers to be used as coatings in gastrointestinal drug releasecarriers to protect some biologically and pharmaceutically activecompounds such as drugs from degrading in the acidic environment of thestomach. The copolymers of the present invention having a relativelyhigh concentration of pendent carboxylic acid groups are stable andwater insoluble in acidic environments but dissolve/degrade rapidly whenexposed to neutral or basic environments. By contrast, copolymers of lowacid to ester ratios are more hydrophobic and will not degrade/resorbrapidly in either basic or acidic environments. Therefore, anotheraspect of the present invention provides a controlled drug deliverysystem in which a biologically or pharmaceutically active agent isphysically coated with a polymer of the present invention having freecarboxylic acid groups.

Other features of the present invention will be pointed out in thefollowing description and claims, which disclose the principles of theinvention and the best modes which are presently contemplated forcarrying them out.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention introduces a novel class of monomers andcopolymers polymerized therefrom in which amino acids or amino acidstructural derivatives are linked together to form new monomers are thenpolymerized to form the new, useful polymers depicted in formula (I).The diphenol monomers of formula IV are prepared following standardprocedures of peptide chemistry such as disclosed in J. P. Greensteinand M. Winitz, Chemistry of the Amino Acids, (John Wiley & Sons, NewYork 1961) and Bodanszky, Practice of Peptide Synthesis(Springer-Verlag, New York, 1984).

Specifically, carbodiimide-mediated coupling reactions in the presenceof hydroxybenzotriazole according to the procedure disclosed in U.S.Pat. No. 5,587,507 and U.S. Pat. No. 5,670,602, the disclosures of bothof which are hereby incorporated by reference. Suitable carbodiimidesare disclosed therein. The preferred carbodiimide is1-ethyl-3-(3-dimethylamino-propyl)carbodiimide hydro-chloride(EDCI.HCl). The crude monomers can be recrystallized twice, first from50% acetic acid and water and then from a 20:20:1 ratio of ethylacetate, hexane and methanol, or, alternatively, flash chromatography onsilica gel is used, employing a 100:2 mixture of methylenechloride:methanol as the mobile phase.

Thioamide monomers (Z⁵=S) can be prepared using the method described byA. Kjaer (Acta Chemica Scandinavica, 6, 1374-83 (1952)). The amide groupin the monomers or polymers can also be converted to thioamide groupsusing the fluorous analog of the Lawesson's reagent (f₆LR) whosestructure appears below (Kaleta, Z., et al., Org. Lett., 8(8), 1625-1628(2006)). The second method is preferable, since it allows the formationof the monomer first then allows the conversion of the amide group tothe thioamide group.

Treatment of an amide with this reagent in 1:1 molar ratio in THF givesthe corresponding thioamide in >88% yield after purification bychromatography or other means.

For the conversion of the tyrosine derived amide monomers to thecorresponding thioamides, the phenolic groups of the monomers are firstprotected by converting them to the diacetyl esters as shown for I₂DTEby treating with Ac₂O/pyridine. The O-protected I₂DTE is then reactedwith f₆LR followed by base hydrolysis to the thioamide—I₂DTE as shown inthe scheme. The transformation can also be carried out on the polymerusing similar procedure.

The N-substituted monomers and polymers of the present invention can beprepared by substituting commercially-available N-substituted startingmaterials for the starting materials of monomers containing amidegroups, such as the monomers disclosed by U.S. Pat. No. 5,099,060, or byN-substituting monomers containing amide groups, such as the monomersprepared according to U.S. Pat. No 5,099,060 using non-N-substitutedstarting materials. There are several methods described in thescientific literature that accomplishes such conversions. For example,the acidic hydrogens of amide groups can be replaced by alkyl groups inthe monomer by reacting the monomer or polymer with paraformaldehydefollowed by hydrogenation using Pd/C/H2 or using sodiumcyanoborohydride.

Provided herein is a method for making N-alkyl/N-aryl monomer precursorsof formula AA-1. Those having ordinary skill in the art, guided by thedisclosure herein, can use the N-alkylation/N-arylation steps of forminga monomer precursor described herein to create anyN-alkylated/N-arylated monomer that corresponds to the polymersdescribed above.

N-Substituted Monomer Preparation

The monomer precursors of formula AA-1 are readily prepared via severaldivergent synthetic routes with the particular route selected relativeto the ease of compound preparation, the commercial availability ofstarting materials, and the like. In some embodiment, the compounds offormula AA-1 can be synthesized as disclosed in U.S. Pat. No. 6.096,782to Audia et al.; Aurelio et al. (Aurelio et al. “Synthetic Preparationof N-Methyl-α-amino Acids”, Chem. Rev., 2004, 5823-5846); Fukuyama etal. (Fukuyama et al. “2,4-Dinitrobenzenesulfonamides: A Simple andPractical Method for the Preparation of a Variety of Secondary Aminesand Diamines”, Tet. Lett., 1997, 5831-5834); and Ma et al. (Ma et al.,“CuI-Catalyzed Coupling Reaction of β-Amino Acids or Esters with ArylHalides at Temperature Lower Than That Employed in the Normal UllmannReaction. Facile Synthesis of SB-214857”, Org. Lett., 3 (16), 2001,2583-2586), the contents of each reference are hereby incorporated byreference in their entirety. For example, the monomer precursors offormula AA-1 can be synthesized as shown in Schemes 8 and 9 below. Othernon-limiting methods for synthesizing the precursors of formula AA-1 areshown below. The ubiquitousness of modified amino acids in theliterature will lead one of skill in the art to a variety of additionalmethods to prepare N-modified amino acids.

In an embodiment, in monomer precursor of formula AA-1, variable R^(A)can be a protected or unprotected side chain of an amino acid. Forexample, R^(A) can be the side chain of Alanine, Cysteine, Glycine,Histidine, Isoleucine, Phenylalanine, Serine, Threonine, Tryptophan,Tyrosine, and Valine. In an exemplary embodiment, R^(A) can be the sidechain of Tyrosine where the phenolic hydroxy is protected. For example,the phenolic hydroxy group of Tryptophan can be protected as a methylether as shown in the precursor of formula AA-W below.

In an embodiment, in the monomer precursor of formula AA-1, variableR^(B) can be an optionally substituted alkyl or aryl substituent. Forexample, R^(B) can be branched or unbranched C₁-C₃₀ alkyl or optionallysubstituted C₆-C₃₀ aryl.

In an embodiment, a monomer precursor for formula AA-1 is given by 8-A:

wherein variable X can be Cl, Br, I, tosylate, mesylate, triflate andthe like.

Synthetic Schemes: N-Alkyl Monomer Precursors

Scheme 8 Route 1

In an embodiment, a method of introducing the N substituent R^(B) ofmonomer precursor AA-1 via a substitution reaction, wherein R^(A) andR^(B) is defined as above, and X can be Cl, Br, tosylate, or mesylatedefined as above, can be accomplished as shown in Scheme 8 route 1. Forexample, in compound 8-A the variable X is a good leaving group and canbe substituted with the appropriate aryl or alkyl amine (8-B) to affordmonomer precursor AA-1 as described in U.S. Pat. No. 3,598,859, which ishereby incorporated by reference in its entirety. Additionally, suitableester derivatives of 8-A can be used with this method.

In some embodiments, coupling of 8-A with a primary, aryl, orheteroarylamine of the formula 8-B under appropriate conditions canprovide AA-1. This reaction is described by, for example, U.S. Pat. No.3,598,859. In an embodiment, the reaction proceeds by combiningapproximately stoichiometric equivalents of 8-A ,wherein X is Cl, Br, orI, with 8-B in a suitable inert diluent such as water, dimethylsulfoxide(DMSO), or the like. The reaction employs an excess of a suitable basesuch as sodium bicarbonate, sodium hydroxide, etc. to scavenge the acidgenerated by the reaction. The reaction is preferably conducted at fromabout 25° C. to about 100° C. until reaction completion which typicallyoccurs within 1 to about 24 hours. Upon reaction completion, AA-1 can beisolated by conventional methods, such as, precipitation,chromatography, filtration and the like.

Scheme 8 Route 2

In one embodiment, a method of introducing the N substituent R^(B) ofmonomer precursor AA-1 can be accomplished via a reductive aminationreaction, as shown in Scheme 8 route 2, wherein R^(A), R^(B), and X aredefine as above. The α-ketoester 8-C can be treated with the appropriatearyl or alkyl amine (8-B) under reductive amination conditions to affordAA-1 as described in U.S. Pat. No. 3,598,859.

For example, in a exemplary embodiment, approximately stoichiometricamounts of an α-ketoester of formula 8-C and an alkyl or aryl amine ofthe formula 8-B can be combined in a solvent such as methanol, ethanoland the like and reacted under conditions which provide for imineformation (not shown). The in situ formed imine can be then reducedunder conventional conditions by a suitable reducing agent, such assodium cyanoborohydride, H₂/palladium on carbon and the like to form theN-aryl or N-alkyl amino acid ester 8-D. In a typical embodiment, thereducing agent is H₂/palladium on carbon which is incorporated into theinitial reaction medium which permits imine reduction in situ in a onepot procedure to provide for the N-aryl or N-alkyl amino acid ester 8-D.Subsequent hydrolysis of ester 8-D can afford the monomer precursorAA-1. For example, the ester can be hydrolyzed using wet basic methanol.

Scheme 8 Route 3

In one embodiment, a method of introducing the N substituent R^(B) ofmonomer precursor AA-1 can be accomplished via an alkylation reaction ofa compound of the formula 8-E and subsequent transformation as shown inScheme 8 route 3. In some embodiments, R^(A) and X are define as above,R^(B) can be branched or unbranched C₁-C₃₀ alkyl or optionallysubstituted C₆-C₃₀ aryl, R^(F) can be H, C₁-C₆ alkyl or aryl(CH₂)—, andR^(E) can be selected from the group consisting of CF₃C(O)—,Cbz—(Carbobenzyloxy), Boc—(tert-Butoxycarbonyl), tosyl—(toluenesulfonyl)or Nosyl—(2-nitrobenzenesulfonyl or 2-nitrobenzenesulfonyl) group,2,4-dinitrobenzenesulfonyl, and the like. The N-substituted compound offormula 8-E can be treated with an alkylating agent (8-B) under theappropriate conditions to afford 8-G, and the subsequent transformationof 8-G can afford monomer precursor AA-1, as shown below.

For example, in an exemplary embodiment, Aurelio et al. disclosesmethods of preparing N-methyl amino acids, these methods can begenerally used to prepare additional N-substituted amino acids, such asN-methyl, N-ethyl, N-benzyl and the like.

In an embodiment, treatment of 8-E, wherein R^(E) is Cbz- or Boc-; R^(F)is H; and R^(A) is Me or —CH₂Phenyl, with methyl iodide in the presenceof Ag₂O in DMF affords 8-E, wherein R^(B) is methyl and R^(F) is methyl.Subsequent hydrolysis of the methyl ester and removal of the carbamatetype protecting group affords the N-methyl amino acid AA-1. This methodcan be modified to use ethyl iodide in place of methyl iodide to affordthe N-ethyl amino acids of formula AA-1. Additionally, this method canbe applied to polymers following the procedure of Das et al. (Das, etal., “N-methylation of N-acyl oligopeptides”, Biochem. Biophys. Res.Commun. 1967,29,211), the contents of which are hereby incorporated byreference, to afford N-methyl polymers.

In one embodiment, treatment of 8-E, wherein R^(E) is Cbz- or Boc-;R^(F) is H; and R^(A) is Me or —CH₂Phenyl with sodium hydride followedby addition of methyl iodide in DMF/THF at 80° C. for 24 h affords 8-E,wherein R^(B) is methyl and R^(F) is methyl. Subsequent hydrolysis ofthe methyl ester with sodium hydroxide in methanol/THF and then removalof the carbamate type protecting group affords the N-methyl amino acidAA-1. This method can be modified to use ethyl iodide in place of methyliodide to afford the N-ethyl amino acids of formula AA-1. In anotherembodiment this same procedure can be used to alkylate 8-E wherein R^(F)is methyl.

In an embodiment, following the procedure of Belagali et al. (Belagaliet al. “A Highly Efficient Method of N-Methylation For The Amino-AcidDerivatives”, Indian J. Chem. Sect. B, 1995, 34(1), 45), the contents ofwhich are hereby incorporated by reference in their entirety, treatmentof 8-E, wherein R^(E) is Boc-; R^(F) is H; and R^(A) is Me or—CH₂PhenylOH, with sodium hexamethyldisilazane in THF followed byaddition of methyl iodide affords 8-E, wherein R^(B) is methyl and R^(F)is methyl. Subsequent hydrolysis of the methyl ester and then removal ofthe carbamate type protecting group affords the N-methyl amino acidAA-1. This method can be modified to use ethyl iodide in place of methyliodide to afford the N-ethyl amino acids of formula AA-1. In anotherembodiment this same procedure can be used to alkylate 8-E wherein R^(F)is methyl.

In an embodiment, following the procedure of Fukuyama et al., treatmentof 8-E, wherein R^(E) is Nosyl; R^(F) is Methyl; and R^(A) is—CH₂Phenyl, with K₂CO₃ in DMF followed by addition of R^(B)—X, whereinR^(B)—X is propyl iodide, affords 8-E, wherein R^(B) is propyl and R^(F)is methyl. Subsequent hydrolysis of the methyl ester and then removal ofthe carbamate type protecting group can afford the N-propyl amino acidAA-1. This method can be modified to use ethyl iodide in place of propyliodide to afford the N-ethyl amino acids of formula AA-1.

Scheme 8 Route 4

In a typical embodiment, following the procedure of Fukuyama et al., aDMF solution of the amino acid ester of the formula (8-EA) can betreated with ethyl bromide in the presence of K₂CO₃ to afford 8-GA.Subsequently, the 2,4-dinitrobenzenesulfonyl group can be removed andthe ester group can be hydrolyzed to afford AA-1A. For example, the2,4-dinitrobenzenesulfonyl group of can be removed by treatment of 8-GAwith thiophenol and K₂CO₃ in DMF followed hydrolysis of the methyl esterwith NaOH in methanol/THF to afford AA-1A. The N-substituted β-aminoacid (AA-1A) can be converted to the N-substituted β-amino ester bymethods known to one of skill in the art. For example, the N-substitutedβ-amino acid can be treated with HCl in a solvent such as ethanol ormethanol to afford the corresponding ethyl or methyl N-aryl β-aminoesters.

Scheme 8 Route 5

Alternatively, in an exemplary embodiment, a DMF solution of thetert-butyl amino acid ester of the formula (8-EB) can be treated withethyl bromide in the presence of K₂CO₃ to afford 8-GB. Subsequently, the2,4-dinitrobenzenesulfonyl group can be removed to afford the tert-butylester AA-1B. For example, the 2,4-dinitrobenzenesulfonyl group of can beremoved by treatment of 8-GB with thiophenol and K₂CO₃ in DMF to affordAA-1B.

N-Aryl Monomer PrecursorsScheme 9

In one embodiment, monomer precursor AA-1 can be synthesized, whereinR^(B) is an aryl group, such as and X is chloride, bromide, or iodide asshown in Scheme 9. For example, the monomer precursor of formula AA-1can be synthesized by an Ullmann reaction, such as the procedure of Maet al.

In an exemplary embodiment, the amino ester 9-A can be converted to 9-B,as shown in Scheme 9. For example in the presence of phenyl iodide, CuIand K₂CO₃ in DMF at 100° C. In one embodiment, the amino ester 9-C canbe treated with phenyl iodide (8-F), CuI and K₂CO₃ in DMF at 100° C. toafford 9-D, as shown in Scheme 9-A. The N-aryl β-amino acid (9-D) can beconverted to the N-aryl β-amino ester by methods known to one of skillin the art. For example, the N-aryl β-amino acid can be treated with HClin a solvent such as ethanol or methanol to afford the correspondingethyl or methyl N-aryl β-amino esters.

In these synthetic methods, the starting materials can contain a chiralcenter (e.g., alanine) and, when a racemic starting material isemployed, the resulting product is a mixture of diastereomers or R,Senantiomers. Alternatively, a chiral isomer of the starting material canbe employed and, if the reaction protocol employed does not racemizethis starting material, a chiral product is obtained. Such reactionprotocols can involve inversion of the chiral center during synthesis.

Accordingly, unless otherwise indicated, the products of this inventionare a mixture of diastereomers (if two or more chiral centers arepresent) or R,S enantiomers (if only one chiral center is present).Preferably, however, when a chiral product is desired, the chiralproduct corresponds to the L-amino acid derivative. Alternatively,chiral products can be obtained via purification techniques whichseparates diastereomers or enantiomers from a R,S mixture to provide forone or the other stereoisomer. Such techniques are well known in theart.

Polymers according to the present invention contain a plurality ofmonomeric repeating units containing an amide group, wherein the amidegroups are N-substituted, and the N-substituents and degree ofN-substitution are effective to render the polymer processable by adesired processing method. Preferably, the minimum amount ofN-substituted monomer is used. This can range from one to three molepercent to render a non-soluble polymer soluble in a given solvent to upto about 25 mole percent to make the same polymer injection moldable.This can be readily determined by one of ordinary skill in the artwithout undue experimentation.

N-alkyl substituents with one to six carbon atoms are preferred, withN-methyl substituents being more preferred.

The monomer compounds are then polymerized to form tissue compatiblebioerodable polymers for medical uses. The diphenol monomers can be usedin any conventional polymerization process using diphenol monomers,including those processes that synthesize polymers traditionallyconsidered hydrolytically stable and non-biodegradable.

This includes polyesters, polycarbonates, polyiminocarbonates,polyarylates, polyurethanes, polyphosphazine polyphosphonates andpolyethers, as well as random block copolymers of these polymers withpoly(alkylene oxides) as described in U.S. Pat. No. 5,658,995, thedisclosure of which is incorporated herein by reference.

It is also understood that the presentation of the various polymerformulae that polymer structures represented may include homopolymersand heteropolymers, which include stereoisomers. Homopolymer is usedherein to designate a polymer comprised of all the same type ofmonomers. Heteropolymer is used herein to designate a polymer comprisedof two or more different types of monomer, which is also called aco-polymer. A heteropolymer or co-polymer may be of a kind known asblock, random and alternating. Further with respect to the presentationof the various polymer formulae, products according to embodiments ofthe present invention may be comprised of a homopolymer, heteropolymerand/or a blend of such polymers.

Polyiminocarbonates are synthesized from dihydroxy and diphenol monomersvia one of the appropriate methods disclosed by U.S. Pat. No. 4,980,449,the disclosure of which is incorporated by reference. According to onemethod, part of the dihydroxy or diphenol compound is converted to theappropriate dicyanate, then, equimolar quantities of the dihydroxy ordiphenol compound and the dicyanate are polymerized in the presence of astrong base catalyst such as a metal alkoxide or metal hydroxide.

The monomers compounds of Formula I may also be reacted with phosgene toform polycarbonates with —O—C(═O)—O— linkages. The method is essentiallythe conventional method for polymerizing diols into polycarbonates.Suitable processes, associated catalysts and solvents are known in theart and are taught in Schnell, Chemistry and Physics of Polycarbonates,(Interscience, New York 1964), the teachings of which are alsoincorporated herein by reference.

Other methods adaptable for use to prepare polycarbonate polymers of thepresent invention are disclosed in U.S. Pat. Nos. 6,120,491, and6,475,477 the disclosures of which are incorporated herein by reference.Polycarbonates may also be prepared by dissolving the Formula I monomerin methylene chloride containing 0.1M pyridine or triethylamine. Asolution of phosgene in toluene at a concentration between about 10 andabout 25 wt %, and preferably about 20 wt %, is added at a constantrate, typically over about two hours, using a syringe pump or othermeans. The reaction mixture is quenched by stirring with tetrahydrofuran(THF) and water, after which the polymer is isolated by precipitationwith isopropanol (IPA). Residual pyridine (if used) is then removed byagitation of a THF polymer solution with a strongly acidic resin, suchas AMBERLYST 15.

The monomer compounds of formula IV may also be directly reacted withaliphatic or aromatic dicarboxylic acids in the carbodiimide mediatedprocess disclosed by U.S. Pat. No. 5,216,115 using 4-(dimethylamino)pyridinium-p-toluene sulfonate (DPTS) as a catalyst to form thealiphatic or aromatic poly(ester amides). The disclosure of U.S. Pat.No. 5,216,115 is incorporated by reference. Dicarboxylic acids accordingto one embodiment of the present invention have the structure of FormulaV:HOOC—R₅—COOH  (V)

in which, for the aliphatic copolymers, R₅ is selected from saturatedand unsaturated, substituted and unsubstituted alkyl groups containingup to 18 carbon atoms, and preferably from 2 to 12 carbon atoms, andoptionally may also include up to eight N, O, P or S atoms. For thearomatic copolymers, R₃ is selected from aryl and alkylaryl groupscontaining up to 24 carbon atoms and preferably from 13 to 20 carbonatoms, and optionally may also include up to eight N, O, P or S atoms.The N-heteroatoms may be N-substituted to reduce polymer T_(g) and meltviscosity.

The process forms polymers with —O—C(═O)—R₅—C(═O)—O— linkages. R₅ may beselected so that the dicarboxylic acids employed as the startingmaterials are either important naturally-occurring metabolites or highlybiocompatible compounds. Aliphatic dicarboxylic acid starting materialstherefore include the intermediate dicarboxylic acids of the cellularrespiration pathway known as the Krebs Cycle. The dicarboxylic acidsinclude a-ketoglutaric acid, succinic acid, fumaric acid and oxaloaceticacid (R₅ of formula III is —CH₂—CH₂—C(═O)—, —CH₂—CH₂—, —CH═CH— and—CH₂—C(═O)—, respectively).

Another naturally-occurring aliphatic dicarboxylic acid is adipic acid(R₅ is (—CH₂—)₄), found in beet juice. Still yet another biocompatiblealiphatic dicarboxylic acid is sebacic acid (R₅ is (—CH₂—)₈), which hasbeen studied extensively and has been found to be nontoxic as part ofthe clinical evaluation of poly(bis(p-carboxy-phenoxy)propane-co-sebacicacid anhydride) by Laurencin et al., J. Biomed. Mater. Res., 24, 1463-81(1990).

Other biocompatible aliphatic dicarboxylic acids include oxalic acid (R₅is a bond), malonic acid (R₅ is —CH₂—), glutaric acid (R₅ is (—CH₂—)₃),pimelic acid (R₅ is (—CH₂—)₅), suberic acid (R₅ is (—CH₂—)₆) and azelaicacid (R₅ is (—CH₂—)₇). R₅ can thus represent (—CH₂—)_(Q), wherein Q isbetween 0 and 8, inclusive. Among the suitable aromatic dicarboxylicacids are terephthalic acid, isophthalic acid and bis(p-carboxy-phenoxy)alkanes such as bis(p-carboxy-phenoxy) propane.

R₅ can also have the structure of formula VI:—(CH₂—)_(a)O—[(CH₂—)_(a)CHR₄—O—]_(m)(CH₂—)_(a)  (VI)wherein a is from 1 to 3, inclusive, m is from 1 to 500,000, inclusive,and R₄ is hydrogen or a lower alkyl group containing from one to fourcarbon atoms. R₄ is preferably hydrogen, a is preferably 1, and m ispreferably between about 10 and about 100, and more preferably betweenabout 10 and about 50.

The diacids of formula VI are formed by the oxidation of poly(alkyleneoxides) according to well-known methods. One example of such a compoundis biscarboxymethyl poly(ethylene glycol), which is commerciallyavailable.

R₅ can also have the structure of formula VII:—R₃—C(═O)—O[(—CH₂)_(a)—CHR₄—O—]_(m)C(═O)—R₃  (VII)wherein a, m and R₄ and the preferred species thereof are the same asdescribed above with respect to formula VI. R₃ is selected from a bondor straight and branched alkyl and alkylaryl groups containing up to 18carbon atoms.

The dicarboxylic acids of formula VII are poly(alkylene oxides)bis-functionalized with dicarboxylic acids having the structure offormula V wherein R₅ is the same as described above for formula V andpreferably contains up to 12 carbon atoms.

The poly(alkylene oxides) of formula VII that are bis-functionalizedwith dicarboxylic acid are prepared by the reaction of anon-functionalized poly(alkylene oxide) with an excess of either thedicarboxylic acid (mediated by a coupling agent such as dicyclohexylcarbodiimide), the anhydride (e.g. succinic anhydride) in the presenceof pyridine or triethylamine, or a dicarboxylic acid chloride (e.g.adipoyl chloride) in the presence of an acid acceptor liketriethylamine.

Polymers prepared from the formula IV monomeric starting materials ofthe present invention with at least one bromine- or iodine-substitutedaromatic ring are radio-opaque, such as the polymers prepared fromradiopaque diphenol compounds prepared according to the disclosure ofU.S. Pat. No. 6,475,477, as well as he disclosure of co-pending andcommonly-owned U.S. patent application Ser. No. 10/592,202, thedisclosures of both of which are incorporated herein by reference. Theiodinated and brominated diphenol monomers of the present invention canalso be employed as radio-opacifying, biocompatible non-toxic additivesfor other polymeric biomaterials.

Bromine and iodine substituted aromatic monomers of the presentinvention are prepared by well-known iodination and brominationtechniques that can be readily employed by those of ordinary skill inthe art guided by the above referenced granted patent and pendingapplication (now published) without undue experimentation. Thehalogenated aromatic compounds from which the halogenated aromaticmonomers the present invention are prepared undergo ortho-directedhalogenation. The term, “ortho-directed”, is used herein to designateorientation of the halogen atom(s) relative to the phenoxy alcoholgroup.

Random or block copolymers of the formula I polymers of the presentinvention with a poly(alkylene oxide) may be prepared according to themethod disclosed in U.S. Pat. No. 5,658,995, the disclosure of which isalso incorporated by reference. The poly(alkylene oxide) is preferably apoly(ethylene glycol) block/unit typically having a molecular weight ofless than about 10,000 per unit. More typically, the poly(ethyleneglycol) block/unit has a molecular weight less than about 4000 per unit.The molecular weight is preferably between about 1000 and about 2000 perunit.

The molar fraction of poly(ethylene glycol) units in block copolymersmay range from grater than zero to less than 1, and is typically greaterthan zero up to about 0.5, inclusive. More preferably, the molarfraction is less than about 0.25 and yet more preferably, less thanabout 0.1. In a more preferred variations, the molar fraction may varyfrom greater than about 0.001 to about 0.08, and most preferably,between about 0.025 and about 0.035.

Unless otherwise indicated, the molar fractions reported herein arebased on the total molar amount of poly(alkylene glycol) and non-glycolunits in the polymers

Applicants have also recognized that the polymer glass transitiontemperature increases as the degree of halogenation and the molarfraction of free carboxylic acid units increases. Higher weightpercentages of poly(alkylene oxide) are typically used in polymers withhigher levels of iodination and/or with higher molar fractions of freecarboxylic acid units to maintain the polymer glass transitiontemperature within a desired range for the end use application.N-alkylation provides an alternative means for lowering the polymerglass transition temperature so that the amount of poly(alkylene oxide)may be lowered or eliminated without adversely affecting the polymermelt properties. The present invention thus places more tools at thedisposal of the polymer chemist for fine-tuning the physico-mechanicalproperties of the inventive polymers.

The formula I polymers having weight-average molecular weights aboveabout 20,000, and preferably above about 80,000, calculated from gelpermeation chromatography (GPC) relative to polystyrene standards usingtetrahydrofuran (THF) as the eluent without further correction.

The polymers of the present invention are defined as including polymerspolymerized from formula IV monomers having pendent free carboxylic acidgroups (R₈═OH). However, it is not possible to polymerize polymershaving pendent free carboxylic acid groups from corresponding monomerswith pendent free carboxylic acid groups without cross-reaction of thefree carboxylic acid group with the co-monomer. Accordingly, polymers inaccordance with the present invention having pendent free carboxylicacid groups are prepared from homopolymers and copolymers of benzyl andtert-butyl ester monomers of the present invention having the structureof formula IV in which R₈ is a benzyl or tert-butyl group.

The benzyl ester homopolymers and copolymers may be converted tocorresponding free carboxylic acid homopolymers and copolymers throughthe selective removal of the benzyl groups by the palladium catalyzedhydrogenolysis method disclosed by co-pending and commonly owned U.S.Pat. No. 6,120,491, the disclosure of which is incorporated herein byreference.

The tert-butyl ester homopolymers and copolymers may be converted tocorresponding free carboxylic acid homopolymers and copolymers throughthe selective removal of the tert-butyl groups by the acidolyis methoddisclosed by the above-referenced U.S. patent application Ser. No.10/592,202, also incorporated herein by reference.

The catalytic hydrogenolysis or acidolysis is necessary because thelability of the polymer backbone prevents the employment of harsherhydrolysis techniques.

Applicants have recognized that the molar fraction of free carboxylicacid units in the polymers of the present invention can be adjustedaccording to the present invention to likewise adjust thedegradation/resorbability of devices made from such polymers. Forexample, applicants have recognized that while poly(DTE-co-35 mol % DTcarbonate), (a tyrosine-derived polycarbonate comprising about 35% freecarboxylic acid units) is 90% resorbed in about 15 days, polycarbonateswith lower amounts of free carboxylic acid will have desirably longerlifetimes in the body. Furthermore, by otherwise adjusting the amount offree carboxylic acid in the polymers across the range of preferred molarfraction, the resulting polymers can be adapted for use in variousapplications requiring different device lifetimes. In general, thehigher the molar fraction of free carboxylic acid units, the shorter thelifetime of the device in the body and more suitable such devices arefor applications wherein shorter lifetimes are required. In certainembodiments where lifetimes of 6 months or more are required, polymersof the presently preferred ranges of free carboxylic acid units tend tobe desirable.

The present invention also includes N-substituted versions of themonomers and polymers of Pacetti, U.S. Pat. Application Pub. No.2006-0115449, incorporated by reference herein in its entirety, preparedaccording to the N-substitution methods disclosed herein.

After polymerization, appropriate work up of the polymers in accordancewith preferred embodiments of the present invention may be achieved byany of a variety of known methods commonly employed in the field ofsynthetic polymers to produce a variety of useful articles with valuablephysical and chemical properties, all derived from tissue compatiblemonomers. The useful articles can be shaped by conventionalpolymer-forming techniques such as extrusion, compression molding,injection molding, solvent casting, spin casting, wet spinning,combinations of two or more thereof, and the like. Shaped articlesprepared from the polymers are useful, inter alia, as degradablebiomaterials for medical implant applications. Such applications includethe use of shaped articles as vascular grafts and stents.

Stent fabrication processes may further include two-dimensional methodsof fabrication such as cutting extruded sheets of polymer, via lasercutting, etching, mechanical cutting, or other methods, and assemblingthe resulting cut portions into stents, or similar methods ofthree-dimensional fabrication of devices from solid forms. In certainother embodiments, the polymers are formed into coatings on the surfaceof an implantable device, particularly a stent, made either of a polymerof the present invention or another material, such as metal. Suchcoatings may be formed on stents via techniques such as dipping, spraycoating, combinations thereof, and the like. Further, stents may becomprised of at least one fiber material, curable material, laminatedmaterial, and/or woven material. Details of stent products andfabrication in which the polymers of the present invention may beemployed are disclosed in co-pending and commonly-owned U.S. patentapplication Ser. No. 10/952,202 filed Sep. 27, 2004, the disclosure ofwhich is incorporated by reference. Stents are preferably fabricatedfrom the radiopaque polymers of the present invention, to permitfluoroscopic positioning of the device.

The highly beneficial combination of properties associated with thepreferred polymers in accordance with embodiments of the presentinvention are well-suited for use in producing a variety of medicaldevices besides stents, especially implantable medical devices that arepreferably radiopaque, biocompatible, and have various times ofbioresorption. For example, applicants have recognized that, in certainembodiments, the polymers are suitable for use in producing implantabledevices for orthopedics, tissue engineering, dental applications, woundclosure, gastric lap bands, drug delivery, cancer treatment, othercardiovascular applications, non-cardiovascular stents such as biliary,esophagus, vaginal, lung-trachea/bronchus, and the like. In addition,the polymers are suitable for use in producing implantable, radiopaquediscs, plugs, and other devices used to track regions of tissue removal,for example, in the removal of cancerous tissue and organ removal, aswell as, staples and clips suitable for use in wound closure, attachingtissue to bone and/or cartilage, stopping bleeding (homeostasis), tuballigation, surgical adhesion prevention, and the like. Applicants havealso recognized that the polymers of the present invention arewell-suited for use in producing a variety of coatings for medicaldevices, especially implantable medical devices.

Furthermore, in some preferred embodiments, the present polymers may beadvantageously used in making various orthopedic devices including, forexample, radiopaque biodegradable screws (interference screws),radiopaque biodegradable suture anchors, and the like for use inapplications including the correction, prevention, reconstruction, andrepair of the anterior cruciate ligament (ACL), the rotator cuff/rotatorcup, and other skeletal deformities.

Other devices, which can be advantageously formed from the polymers ofthe present invention, include devices for use in tissue engineering.Examples of suitable devices include tissue engineering scaffolds andgrafts (such as vascular grafts, grafts or implants used in nerveregeneration). The present polymers may also be used to form a varietyof devices effective for use in closing internal wounds. For example,biodegradable sutures, clips, staples, barbed or mesh sutures,implantable organ supports, and the like, for use in various surgery,cosmetic applications, and cardiac wound closures can be formed.

Various devices finding use in dental applications may advantageously beformed according to preferred aspects of the present invention. Forexample, devices for guided tissue regeneration, alveolar ridgereplacement for denture wearers, and devices for the regeneration ofmaxilla-facial bones may benefit from being radiopaque so that thesurgeon/dentist can ascertain the placement and continuous function ofsuch implants by simple X-ray imaging.

The present polymers are also useful in the production of gastric lapbands for use in the treatment of obesity. The production of radiopaquelap bands allows for more effective monitoring of the devices in thehuman body, and more effective treatment of obesity.

In addition to intravascular stents and non-cardiovascular stents, thepresent polymers are useful in a number of other cardiovascular andvascular devices. For example, valves, chordae tendinea replacements,annuloplasty rings, leaflet repair patches, vascular grafts, vasculartubes, patches for septal defects, arterial and venous access closuredevices (plugs), and the like can be formed for use in replacementrepair of heart valves, tubes, and the like. In addition, portions of anartificial heart, such as the rough surface/fibroid layer (bellow pumps)may be formed from the polymers of the instant invention.

The polymers of the present invention are also useful in the productionof bioresorbable, inherently radiopaque polymeric embolotherapy productsfor the temporary and therapeutic restriction or blocking of bloodsupply to treat tumors and vascular malformations, e.g., uterinefibroids, tumors (i.e., chemoembolization), hemorrhage (e.g., duringtrauma with bleeding) and arteriovenous malformations, fistulas andaneurysms delivered by means of catheter or syringe. Details ofembolotherapy products and methods of fabrication in which the polymersof the present invention may be employed are disclosed in co-pending andcommonly-owned U.S. patent application Ser. No. 10/952,274 filed Sep.27, 2004, the disclosure of which is incorporated by reference.Embolotherapy treatment method are by their very nature local ratherthan systemic and the products are preferably fabricated from theradiopaque polymers of the present invention, to permit fluoroscopicmonitoring of delivery and treatment.

The present polymers are further useful in the production of a widevariety of therapeutic agent delivery devices. Such devices may beadapted for use with a variety of therapeutics including, for example,pharmaceuticals (i.e., drugs) and/or biological agents as previouslydefined and including biomolecules, genetic material, and processedbiologic materials, and the like. Any number of transport systemscapable of delivering therapeutics to the body can be made, includingdevices for therapeutics delivery in the treatment of cancer,intravascular problems, dental problems, obesity, infection, and thelike.

In certain embodiments, any of the aforementioned devices describedherein can be adapted for use as a therapeutic delivery device (inaddition to any other functionality thereof). Controlled therapeuticdelivery systems may be prepared, in which a therapeutic agent, such asa biologically or pharmaceutically active and/or passive agent, isphysically embedded or dispersed within a polymeric matrix or physicallyadmixed with a polymer of the present invention. Controlled therapeuticagent delivery systems may also be prepared by direct application of thetherapeutic agent to the surface of an implantable medical device suchas a bioresorbable stent device (comprised of at least one of thepresent polymers) without the use of these polymers as a coating, or byuse of other polymers or substances for the coating.

The Q¹ pendant groups of the polymers of the present invention may alsobe derivatized by the covalent attachment of a therapeutic agent.Depending upon whether Q¹ defines a free carboxylic acid, a carboxylicacid amide, a hydroxyl group, or the like, and depending upon themoieties present on the underivaitized therapeutic agent, the covalentbond may be an amide bond or an ester bond. Typically, the therapeuticagent is derivatized at a primary or secondary amine, hydroxyl, ketone,aldehyde or carboxylic acid group. Chemical attachment procedures aredescribed by U.S. Pat. Nos. 5,219,564 and 5,660,822; Nathan et al., Bio.Cong. Chem., 4, 54-62 (1993) and Nathan, Macromolecules, 25, 4476(1992), the disclosures of which are incorporated by reference. Thetherapeutic agent may first be covalently attached to a monomer, whichis then polymerized, or the polymerization may be performed first,followed by covalent attachment of the therapeutic agent.

Hydrolytically stable conjugates are utilized when the therapeutic agentis active in conjugated form. Hydrolyzable conjugates are utilized whenthe therapeutic agent is inactive in conjugated form.

Therapeutic agent delivery compounds may also be formed by physicallyblending the therapeutic agent to be delivered with the polymers of thepresent invention using conventional techniques well-known to those ofordinary skill in the art. For this therapeutic agent deliveryembodiment, it is not essential that the polymer have pendent groups forcovalent attachment of the therapeutic agent.

The polymer compositions of the present invention containing therapeuticagents, regard-less of whether they are in the form of polymerconjugates or physical admixtures of polymer and therapeutic agent, aresuitable for applications where localized delivery is desired, as wellas in situations where a systemic delivery is desired. The polymerconjugates and physical admixtures may be implanted in the body of apatient in need thereof, by procedures that are essentially conventionaland well-known to those of ordinary skill in the art.

Implantable medical devices may thus be fabricated that also serve todeliver a therapeutic agent to the site of implantation by beingfabricated from or coated with the therapeutic agent delivery system ofthe present invention in which a polymer of the present invention has atherapeutic agent physically admixed therein or covalently bondedthereto, such as a drug-eluting stent. Embolotherapeutic particles mayalso be fabricated for delivery of a therapeutic agent.

Examples of biologically or pharmaceutically active therapeutic agentsthat may by covalently attached to the polymers of the present inventioninclude acyclovir, cephradine, malphalen, procaine, ephedrine,adriamycin, daunomycin, plumbagin, atropine, quinine, digoxin,quinidine, biologically active peptides, chlorin e.sub.6, cephradine,cephalothin, proline and proline analogs such as cis-hydroxy-L-proline,malphalen, penicillin V, aspirin and other non-steroidalanti-inflammatories, nicotinic acid, chemodeoxycholic acid,chlorambucil, anti-tumor and anti-proliferative agents, includinganti-proliferative agents that prevent restenosis, hormones such asestrogen, and the like. Biologically active compounds, for the purposesof the present invention, are additionally defined as including cellattachment mediators, biologically active ligands, and the like.

The invention described herein also includes various pharmaceuticaldosage forms containing the polymer-therapeutic agent combinations ofthe present invention. The combination may be a bulk matrix forimplantation or fine particles for administration by traditional means,in which case the dosage forms include those recognized conventionally,e.g. tablets, capsules, oral liquids and solutions, drops, parenteralsolutions and suspensions, emulsions, oral powders, inhalable solutionsor powders, aerosols, topical solutions, suspensions, emulsions, creams,lotions, ointments, transdermal liquids and the like.

The dosage forms may include one or more pharmaceutically acceptablecarriers. Such materials are non-toxic to the recipients at the dosagesand concentrations employed, and include diluents, solubilizers,lubricants, suspending agents, encapsulating materials, penetrationenhancers, solvents, emollients, thickeners, dispersants, buffers suchas phosphate, citrate, acetate and other organic acid salts,anti-oxidants such as ascorbic acid, preservatives, low molecular weight(less than about 10 residues) peptides such as polyarginine, proteinssuch as serum albumin, gelatin, or immunoglobulins, other hydrophilicpolymers such as poly(vinylpyrrolidinone), amino acids such as glycine,glutamic acid, aspartic acid, or arginine, monosaccharides,disaccharides, and other carbohydrates, including cellulose or itsderivatives, glucose, mannose, or dextrines, chelating agents such asEDTA, sugar alcohols such as mannitol or sorbitol, counterions such assodium and/or nonionic surfactants such as tween, pluronics or PEG.

The therapeutic agents to be incorporated in the polymer conjugates andphysical admixtures of this invention may be provided in aphysiologically acceptable carrier, excipient stabilizer, etc., and maybe provided in sustained release or timed release formulationssupplemental to the polymeric formulation prepared in this invention.Liquid carriers and diluents for aqueous dispersions are also suitablefor use with the polymer conjugates and physical admixtures.

Subjects in need of treatment, typically mammalian, using thepolymer-therapeutic agent combinations of this invention, can beadministered dosages that will provide optimal efficacy. The dose andmethod of administration will vary from subject to subject and bedependent upon such factors as the type of mammal being treated, itssex, weight, diet, concurrent medication, overall clinical condition,the particular compounds employed, the specific use for which thesecompounds are employed, and other factors which those skilled in themedical arts will recognize. The polymer-therapeutic agent combinationsof this invention may be prepared for storage under conditions suitablefor the preservation of therapeutic agent activity as well asmaintaining the integrity of the polymers, and are typically suitablefor storage at ambient or refrigerated temperatures.

Aerosol preparations are typically suitable for nasal or oralinhalation, and may be in powder or solution form, in combination with acompressed gas, typically compressed air. Additionally, aerosols may beused topically. In general, topical preparations may be formulated toenable one to apply the appropriate dosage to the affected area oncedaily, and up to three to four times daily, as appropriate.

Depending upon the particular compound selected, transdermal deliverymay be an option, providing a relatively steady delivery of the drug,which is preferred in some circumstances. Transdermal delivery typicallyinvolves the use of a compound in solution, with an alcoholic vehicle,optionally a penetration enhancer, such as a surfactant, and otheroptional ingredients. Matrix and reservoir type transdermal deliverysystems are examples of suitable transdermal systems. Transdermaldelivery differs from conventional topical treatment in that the dosageform delivers a systemic dose of the therapeutic agent to the patient.

The polymer-drug formulations of this invention may also be administeredin the form of liposome delivery systems, such as small unilamellarvesicles, large unilamellar vesicles and multilamellar vesicles.Liposomes may be used in any of the appropriate routes of administrationdescribed herein. For example, liposomes may be formulated that can beadministered orally, parenterally, transdermally, or via inhalation.Therapeutic agent toxicity could thus be reduced by selective deliveryto the affected site. For example, if the therapeutic agent is liposomeencapsulated, and is injected intravenously, the liposomes used aretaken up by vascular cells and locally high concentrations of thetherapeutic agent could be released over time within the blood vesselwall, resulting in improved action of the therapeutic agent. Theliposome encapsulated therapeutic agents are preferably administeredparenterally, and particularly, by intravenous injection.

Liposomes may be targeted to a particular site for release of thetherapeutic agent. This would obviate excessive dosages that are oftennecessary to provide a therapeutically useful dosage of a therapeuticagent at the site of activity, and consequently, the toxicity and sideeffects associated with higher dosages.

The therapeutic agents incorporated into the polymers of this inventionmay desirably further incorporate agents to facilitate their deliverysystemically to the desired target, as long as the delivery agent meetsthe same eligibility criteria as the therapeutic agents described above.The active therapeutic agents to be delivered may in this fashion beincorporated with antibodies, antibody fragments, growth factors,hormones, or other targeting moieties, to which the therapeutic agentmolecules are coupled.

The polymer-therapeutic agent combinations of this invention may also beformed into shaped particles, such as valves, stents, tubing,prostheses, and the like.

Therapeutically effective dosages may be determined by either in vitroor in vivo methods. For each particular compound of the presentinvention, individual determinations may be made to determine theoptimal dosage required. The range of therapeutically effective dosageswill naturally be influenced by the route of administration, thetherapeutic objectives, and the condition of the patient. For thevarious suitable routes of administration, the absorption efficiencymust be individually determined for each drug by methods well known inpharmacology. Accordingly, it may be necessary for the therapist totiter the dosage and modify the route of administration as required toobtain the optimal therapeutic effect. The determination of effectivedosage levels, that is, the dosage levels necessary to achieve thedesired result, will be within the ambit of one skilled in the art.Typically, applications of compound are commenced at lower dosagelevels, with dosage levels being increased until the desired effect isachieved. The release rate from the formulations of this invention arealso varied within the routine skill in the art to determine anadvantageous profile, depending on the therapeutic conditions to betreated.

A typical dosage might range from about 0.001 mg/k/g to about 1,000mg/k/g, preferably from about 0.01 mg/k/g to about 100 mg/k/g, and morepreferably from about 0.10 mg/k/g to about 20 mg/k/g. Advantageously,the compounds of this invention may be administered several times daily,and other dosage regimens may also be useful.

In practicing the methods of this invention, the polymer-therapeuticagent combinations may be used alone or in combination with othertherapeutic or diagnostic agents. The compounds of this invention can beutilized in vivo, ordinarily in mammals such as primates such as humans,sheep, horses, cattle, pigs, dogs, cats, rats and mice, or in vitro.

One major advantage of using the radiopaque, bioresorbable polymers ofthe instant invention in therapeutic agent delivery applications is theease of monitoring the release of a therapeutic agent and the presenceof the implantable therapeutic delivery system. Because the radiopacityof the polymeric matrix is due to covalently attached halogensubstituents, the level of radiopacity is directly related to theresidual amount of the degrading therapeutic agent delivery matrix stillpresent at the implant site at any given time after implantation. Inpreferred embodiments, the rate of therapeutic release from thedegrading therapeutic delivery system will be correlated with the rateof polymer resorption. In such preferred embodiments, thestraightforward, quantitative measurement of the residual degree ofradio-opacity will provide the attending physician with a way to monitorthe level of therapeutic release from the implanted therapeutic deliverysystem.

The following non-limiting examples set forth herein below illustratecertain aspects of the invention. All parts and percentages are by molepercent unless otherwise noted and all temperatures are in degreesCelsius unless otherwise indicated. All solvents were HPLC grade and allother reagents were of analytical grade and were used as received,unless otherwise indicated.

EXAMPLES Example 1 N-Alkyl Substitution

In a pressure vessel compound 8-EB is dissolved in DMF with K₂CO₃ (2equiv) at room temperature and then treated with ethyl bromide (1.1equiv.) dropwise via syringe. The pressure vessel is then sealed and thereaction is heated to 60° C., at 30 min intervals the reaction isallowed to cool to room temperature and the progress is checked by TLC(thin layer chromatography) or LC/MS. The reaction is quenched withwater and the aqueous layer is extracted. The organic layer is driedover Na₂SO₄, filtered and the solvent removed under reduced pressure toafford 8-GB. The intermediate 8-GB is dissolved in DMF in the presenceof excess K₂CO₃, then thiophenol is added and the mixture is stirred atroom temperature until the completion of the reaction as indicated byTLC. The solid is removed by filtration and the solvent is removed underreduced pressure. The crude mixture is then dissolved in wetmethanol/THF in the presence of catalytic NaOH, upon completion of thehydrolysis of the ester the solvent is removed under reduced pressure.The residue was dissolved in water, acidified to pH 5, and extractedwith ethyl acetate to afford AA-1B.

Example 2 N-Aryl Substitution

To a solution of phenyl iodide (1 mmol) and β-amino ester (9-C) (1 mmol)in DMF (5 mL) is added potassium carbonate (2.5 mmol), 0.1 mL of water,and CuI (0.1 mmol) under nitrogen. After the mixture is stirred at 100°C. for 48 h under nitrogen atmosphere, the cooled solution isconcentrated in vacuo. The residue is dissolved in water, acidified topH 5, and extracted with ethyl acetate. The combined organic layers areconcentrated and purified by chromatography to afford the correspondingN-aryl β-amino acid (9-D).

The N-aryl β-amino acid (9-D) can be converted to the N-aryl β-aminoacid by methods known to one of skill in the art. For example, theN-aryl β-amino acid can be treated with HCl in a solvent such as ethanolor methanol to afford the corresponding ethyl or methyl N-aryl β-aminoesters.

Example 3 N-Substituted Monomers

Monomer PP-IA:

can be synthesized from the monomer precursor of formula AA-1. In atypical embodiment, as shown in Scheme 10, the polymerization precursor10-C can be synthesized from AA-1B. Iodination of3-(4-hydroxyphenyl)propionic acid (10-A) affords3-(4-hydroxy-3,5-diiodophenyl)propanoic acid. Subsequent coupling of10-B with AA-1B followed by removal of the phenol protecting groupafford the polymerization precursor 10-C. For example, treatment of3-(4-hydroxyphenyl)propionic acid (10-A) with chloroiodide affords3-(4-hydroxy-3,5-diiodophenyl)propanoic acid (10-B). Coupling of 10-Bwith AA-1B using N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimidehydrochloride (EDCI) followed by deprotection of the phenol protectinggroup can afford the polymerization precursor 10-C. The removal of themethyl protecting group can be accomplished using boron tribromide(BBr₃) in methylene chloride (DCM). The polymerization precursor 10-Ccan be converted into polymeric form following the methods disclosed insynthetic scheme 1-6. Additional monomer subunits can be synthesizedfrom monomer precursors of formula AA-1 following the method of Scheme10 with appropriate modifications readily apparent to one of skill inthe art.

Example 4 Preparation of Di-Iodinated Aromatic Hydroxy Acids

A 2 M solution of KICl2 was prepared using a literature procedure.¹ In a2 L beaker were stirred place 166.2 g (1.0 mole) of DAT and 800 mL2-propanol. To the resulting solution was added 158 g (2.0 mole) ofpyridine and 1 L (2.0 moles) of 2 M solution of KICl₂. After 1 h ofstirring 3 L of water was added to the reaction mixture, and the productthat precipitated was collected by filtration and washed with water. Forfurther purification the crude product was dissolved in 4 L of watercontaining 80 g (2.0 mol) of sodium hydroxide and filtered. The filtratewas cooled to room temperature and acidified with acetic acid to a pH of5.5. The product was isolated by filtration and washed with severalportions of water and then dried under vacuum for constant weight whichgave 375 g (90% yield) of 3-(3,5-diiodo-4-hydroxyphenyl)propionic acid(I₂DAT). Using similar procedures 4-hydroxyphenyl acetic acid and4-hydroxy benzoic acid were iodinated to the corresponding di-iodinatedcompounds.

Example 5 Synthesis of I₂DAT-NMeTyr-OMe Monomer

A diphenolic monomer was prepared by coupling the3-(3,5-diiodo-4-hydroxyphenyl)propionic acid (I₂DAT) of Example 1 withN-methyl tyrosine methyl ester HCl salt (NMeTM.HCl) (Bache Biosciences,Inc. King of Prussia, Pa.) using 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) as the coupling agent. In particular,1.6 g (3.8 mmol) of I₂DAT, 0.99 g (4.02 mmol) of NMeTM.HCl 54 mg (0.40mmol) of hydroxybenzotriazole and 20 mL of tetrahydrofuran was stirredin a 50 mL round-bottomed flask at 0-5° C. To the flask was then addedEDC (0.81 g, 4.2 mmol).

The reaction mixture was stirred at 0 to 5° C. for 1 h and then at roomtemperature for 3 h. Most of the THF was evaporated off and the reactionmixture was stirred with 50 mL of ethyl acetate and 50 mL 0.2 M HCl. Thelayers were separated using a separatory funnel. The organic layer waswashed with 3×25 mL of 0.2 M HCl, 3×25 mL of 5% sodium bicarbonatesolution and 25 mL of 20% NaCl. The organic layer was then concentratedwhen an oil was obtained. The product was identified as N-methyl I₂DTEby ¹H NMR and elemental analysis. HPLC gave a peak with expectedretention time along with a minor byproduct which is normally presenteven when simple tyrosine esters are used.

Example 6 Polymerization of NMe-I₂DTE

The diphenolic monomer NMe-I₂DTE of Example 2 was polymerized to form apolycarbonate using the standard phosgenation procedure disclose by U.S.Pat. No. 5,099,060.

Example 7 Synthesis of Monomers of N-Alkyl Tyrosine Esters

Preparation of diphenolic monomers from N-alkyl tyrosine esters isexemplified by the synthesis of N-ethyl I₂DTE. In particular, 1.6 g (3.8mmol) of I₂DAT, 1.10 g (4.02 mmol) of N-ethyl tyrosine ethyl ester HClsalt (NEtTE.HCl—prepared by the method disclosed by Aureilo, et al.,Chem. Rev., 104, 5823-46 (2004), the entire disclosure of which isincorporated by reference), 54 mg (0.40 mmol) of hydroxybenzotriazoleand 20 mL of tetrahydrofuran is stirred in a 50 mL round-bottomed flaskat 0-5° C. To the flask is then added EDC (0.81 g, 4.2 mmol). Thereaction mixture is stirred at 0 to 5° C. for 1 h and then at roomtemperature for 3 h. Most of the THF is evaporated off and the reactionmixture is stirred with 50 mL of ethyl acetate and 50 mL 0.2 M HCl. Thelayers are separated using a separatory funnel. The organic layer iswashed with 3×25 mL of 0.2 M HCl, 3×25 mL of 5% sodium bicarbonatesolution and 25 mL of 20% NaCl. The organic layer is then concentratedwhen an oil is obtained. The product is characterized by ¹H NMR,elemental analysis and HPLC.

Example 8 Thioamide Synthesis and N-Alkylation

In a Schlenck tube are placed diacetyl-I₂DTE (693 mg, 1.0 mmol), f₆LR(1.13 g, 1.0 mmol), and 20 mL of THF. The Schlenck tube is heated in anoil bath 55° C. for 4 h. To the reaction mixture is then added 10 g ofalumina and the solvent was removed by evaporation. The crude product ispurified by short column packed with fluorous reverse phase silica. Theproduct is then subjected to hydrolysis with dilute sodium hydroxidefollowed by acidification to give the I₂DTE-thioamide. The compound isthen N-methylated according to Example 7.

The description of the preferred embodiments should be taken asillustrating, rather than as limiting, the present invention as definedby the claims. As will be readily appreciated, numerous combinations ofthe features set forth above can be utilized without departing from thepresent invention as set forth in the claims. Such variations are notregarded as a departure from the spirit and scope of the invention, andall such modifications are intended to be included within the scope ofthe following claims.

1. A blood-contacting or tissue-implantable medical device formed from a biocompatible, bioresorbable polymer comprising a plurality of monomeric repeating units containing an amide group, wherein said amide groups comprise N-substituted amide groups and the N-substituents and degree of N-substitution are effective to lower the melt viscosity of said polymer, the solution viscosity of said polymer, or both, compared to the same polymer without N-substitution.
 2. The medical device according to claim 1, wherein said N-substituents and degree of N-substitution are effective to reduce the melt viscosity of said polymer, the solution viscosity of said polymer, or both, at least about 5%.
 3. The medical device according to claim 1, wherein said N-substituents of said polymer comprise C₁-C₆ alkyl groups.
 4. A biocompatible, bioresorbable polymer comprising one or more recurring units of the formula:

wherein X¹ and X² are each independently selected from Br and I; y1 and y2 are each independently zero or an integer in the range of 1 to 4, and R¹ is selected from the group consisting of substituted or unsubstituted, saturated or unsaturated, straight chain or branched aliphatic groups containing up to 48 carbon atoms, substituted or unsubstituted aromatic groups containing up to 48 carbon atoms, and substituted or unsubstituted araliphatic groups containing up to 48 carbon atoms in which the aliphatic portions are straight chain or branched and saturated or unsaturated, and R¹ contains from 2 to 8 heteroatoms selected from O, S and N, in which two of the heteroatoms form a polymer backbone amide group, wherein some or all of the recurring units comprise backbone amide groups that are N-substituted with a C₁-C₆ alkyl group; wherein the degree of N-substitution is effective to lower the melt viscosity, the solution viscosity, or both, compared to the same polymer without N-substitution; and wherein said polymer has a weight-average molecular weight above about 20,000.
 5. A biocompatible, bioresorbable polymer comprising one or more recurring units of the formula:

wherein X¹ and X² are each independently selected from Br and I; y1 and y2 are each independently zero or an integer in the range of 1 to 4, and R¹ is selected from the group consisting of substituted or unsubstituted, saturated or unsaturated, straight chain or branched aliphatic groups containing up to 48 carbon atoms, substituted or unsubstituted aromatic groups containing up to 48 carbon atoms, and substituted or unsubstituted araliphatic groups containing up to 48 carbon atoms in which the aliphatic portions are straight chain or branched and saturated or unsaturated, and R1 contains from 2 to 8 heteroatoms selected from O, S and N, in which two of the heteroatoms form a polymer backbone amide group; wherein some or all of the recurring units comprise backbone amide groups that are N-substituted with a C₁-C₆ alkyl group, wherein said R¹ group further comprises poly (alkylene oxide) heteroatoms; and wherein the degree of N-substitution is effective to lower the melt viscosity, the solution viscosity, or both, compared to the same polymer without N-substitution.
 6. The polymer of claim 4, wherein R¹ contains between about 18 and about 36 carbon atoms.
 7. A biocompatible, bioresorbable polymer comprising one or more recurring units of the formula:

wherein X¹ and X² are each independently selected from Br and I; y1 and y2 are each independently zero or an integer in the range of 1 to 4, and R¹ is selected from the group consisting of substituted or unsubstituted, saturated or unsaturated, straight chain or branched aliphatic groups containing up to 48 carbon atoms, substituted or unsubstituted aromatic groups containing up to 48 carbon atoms, and substituted or unsubstituted araliphatic groups containing up to 48 carbon atoms in which the aliphatic portions are straight chain or branched and saturated or unsaturated, and R¹ contains from 2 to 8 heteroatoms selected from O, S and N, in which two of the heteroatoms form a polymer backbone amide group; wherein some or all of the recurring units comprise backbone amide groups that are N-substituted with a C₁-C₆ alkyl group; wherein R¹ is:

in which R¹³ and R¹⁴ each independently contain between 0 and 8 carbons atoms, inclusive, and are independently selected from (—CHR¹)_(e)—CH═CH—(CHR¹—)_(e) and (—CHR¹)_(f)(—CHNQ²)_(g)(—CHR¹)_(f), wherein R¹ is H or lower alkyl, each e independently ranges between 0 and 6, inclusive, each f independently ranges between 0 and 8, inclusive and g is 0 or 1; Z⁷ is O or S; R^(x) is a C₁-C₆ alkyl group; Q¹ is C(═Z⁵)—R⁸, wherein Z⁵ is O or S; Q² is —N(R^(x))₂ or —N(R^(x)Q¹) and R⁸ is selected from the group consisting of H, OH, a therapeutically active moiety, a poly(alkylene oxide), —X₃—C_(i)-C₁₈ alkyl, —X₃-alkenyl, —X₃-alkynyl, —X₅-cycloalkyl, —X₅-heterocyclyl, —X₅-aryl and —X₅-heteroaryl; X₃ is selected from a bond, O, S, and N-alkyl; X₅ is selected from a bond, lower alkyl, O, S and N-alkyl; and wherein the degree of N-substitution is effective to lower the melt viscosity, the solution viscosity, or both, compared to the same polymer without N-substitution.
 8. The polymer according to claim 7, wherein R⁸ is an alkyl-terminated poly(alkylene oxide) selected from the group consisting of methoxy-terminated poly(ethylene glycols) (PEG) of molecular weight 100 to 10,000, methoxy-terminated poly(propylene glycols) (PPG) of molecular weight 100 to 10,000 and methoxy-terminated block copolymers of PEG and PPG of molecular weight 100 to 10,000.
 9. The polymer according to claim 4, wherein at least one monomeric aromatic ring of formula (I) is substituted with iodine, so that the sum of y1 and y2 in formula (I) is greater than zero.
 10. The polymer according to claim 7, wherein R⁸ is OH.
 11. The polymer according to claim 7, wherein R¹ is selected from:

wherein R^(x) is a C₁-C₆ alkyl group; Z⁴ and Z⁵ are each independently O or S; a and b independently range between 0 and 8, inclusive; R⁸ is selected from the group consisting of H, OH, a therapeutically active moiety, a poly(alkylene oxide), —X₃—C₁-C₁₈ alkyl, —X₃-alkenyl, —X₃-alkynyl, —X₅-cycloalkyl, —X₅-hetero-cyclyl, —X₅-aryl and —X₅-heteroaryl; X₃ is selected from a bond, O, S, and N-alkyl; and X₅ is selected from a bond, lower alkyl, O, S and N-alkyl.
 12. A polymer according to claim 4, characterized by being a polycarbonate, polyarylate, polyiminocarbonate, polyphosphazene or polyphosphoester, comprising one or more recurring units having the structure of formula (Ia),

wherein X¹ and X² are each independently selected from Br and I; y1 and y2 are each independently zero or an integer in the range of 1 to 4, and R¹ is selected from the group consisting of substituted or unsubstituted, saturated or unsaturated, straight chain or branched aliphatic groups containing up to 48 carbon atoms, substituted or unsubstituted aromatic groups containing up to 48 carbon atoms, and substituted or unsubstituted araliphatic groups containing up to 48 carbon atoms in which the aliphatic portions are straight chain or branched and saturated or unsaturated, and R¹ contains from 2 to 8 heteroatoms selected from O, S and N, in which two of the heteroatoms form a polymer backbone amide group that is N-substituted with a C₁-C₆ alkyl group and A¹ is selected from the group consisting of:

wherein R¹⁰ is selected from H, C₁-C₃₀ alkyl, alkenyl or alkynyl and C₂-C₃₀ hetero-alkyl; heteroalkenyl or heteroalkynyl, and R¹² is selected from C₁-C₃₀ alkyl, alkenyl or alkynyl, C₁- C₃₀ heteroalkyl; heteroalkenyl or heteroalkynyl, C₅-C₃₀ heteroalkylaryl, heteroalkenylary or heteroalkynylaryl, C₆-C₃₀ alkylaryl, alkenylaryl or alkynylaryl, and C₅-C₃₀ heteroaryl; and wherein the degree of N-substitution is effective to lower the melt viscosity, the solution viscosity, or both, compared to the same polymer without N-substitution.
 13. The polymer according to claim 4, further comprising recurring polyalkylene oxide block units comprising repeating units with the structure:

wherein B is —O—((CHR⁶)_(p)—O)_(q)—; each R⁶ is independently H or C₁ to C₃ alkyl; p is an integer ranging between one and four, inclusive; q is an integer ranging between about five and about 3000; and A² is the same as A¹.
 14. A biocompatible, bioresorbable polymer comprising one or more recurring units of the formula:

wherein X¹ and X² are each independently selected from Br and I; y1 and y2 are each independently zero or an integer in the range of 1 to 4, and R¹ is selected from the group consisting of substituted or unsubstituted, saturated or unsaturated, straight chain or branched aliphatic groups containing up to 48 carbon atoms, substituted or unsubstituted aromatic groups containing up to 48 carbon atoms, and substituted or unsubstituted araliphatic groups containing up to 48 carbon atoms in which the aliphatic portions are straight chain or branched and saturated or unsaturated, and R¹ contains from 2 to 8 heteroatoms selected from O, S and N, in which two of the heteroatoms form a polymer backbone amide group that is N-substituted with a C₁-C₆ alkyl group, and characterized by being a copolymer of a diphenol monomer that is not N-substituted.
 15. An implantable drug delivery device comprising a therapeutically effective amount of a biologically or a physiologically active compound in combination with a biocompatible, bioresorbable polymer comprising a plurality of monomeric repeating units containing an amide group, wherein the amide groups comprise N-substituted amide groups and the N-substituents and degree of N-substitution are effective to lower the melt viscosity of said polymer, the solution viscosity of said polymer, or both, compared to the same polymer without N-substitution.
 16. A polymer according to claim 12, further comprising recurring polyalkylene oxide block units comprising repeating units with the structure:

wherein B is —O—((CHR⁶)_(p)—O)_(q)—; each R⁶ is independently H or C₁ to C₃ alkyl; p is an integer ranging between one and four, inclusive; q is an integer ranging between about five and about 3000; and A² is the same as A¹.
 17. The polymer of claim 12, characterized by being a copolymer of a diphenol monomer that is not N-substituted.
 18. The polymer according to claim 5, wherein at least one monomeric aromatic ring of formula (I) is substituted with iodine, so that the sum of y1 and y2 in formula (I) is greater than zero.
 19. The polymer according to claim 7, wherein at least one monomeric aromatic ring of formula (I) is substituted with iodine, so that the sum of y1 and y2 in formula (I) is greater than zero.
 20. The polymer according to claim 12, wherein at least one monomeric aromatic ring of formula (I) is substituted with iodine, so that the sum of y1 and y2 in formula (I) is greater than zero.
 21. The polymer according to claim 14, wherein at least one monomeric aromatic ring of formula (I) is substituted with iodine, so that the sum of y1 and y2 in formula (I) is greater than zero.
 22. The polymer according to claim 5, further comprising recurring polyalkylene oxide block units comprising repeating units with the structure:

wherein B is —O—((CHR⁶)_(p)—O)_(q)—; each R⁶ is independently H or C₁ to C₃ alkyl; p is an integer ranging between one and four, inclusive; q is an integer ranging between about five and about 3000; and A² is the same as A¹.
 23. The polymer according to claim 7, further comprising recurring polyalkylene oxide block units comprising repeating units with the structure:

wherein B is —O—((CHR⁶)_(p)—O)_(q)—; each R⁶ is independently or C₁ to C₃ alkyl; p is an integer ranging between one and four, inclusive; q is an integer ranging between about five and about 3000; and A² is the same as A¹.
 24. The polymer according to claim 14, further comprising recurring polyalkylene oxide block units comprising repeating units with the structure:

wherein B is —O—((CHR⁶)_(p)—O)_(q)—; each R⁶ is independently H or C₁ to C₃ alkyl; p is an integer ranging between one and four, inclusive; q is an integer ranging between about five and about 3000; and A² is the same as A¹.
 25. The polymer of claim 5, characterized by being a copolymer of a diphenol monomer that is not N-substituted.
 26. The polymer of claim 7, characterized by being a copolymer of a diphenol monomer that is not N-substituted. 